Resonant window assembly



Oct.'18, 1960 H. c. ALEXANDER REsoNANT WINDOW ASSEMBLY Filed 0G12.l 8, 1958 INV EN TOR. HERBERT c. ALEXANDER BY M ATTQRNEY RESONANT WINDOW ASSEMBLY Herbert C. Alexander, Peabody, Mass., assignor to Bomac Laboratories Inc., Beverly, Mass., a corporation of Massachusetts Filed Oct. 8, 1958, Ser. No. 766,079

3 Claims. (Cl. 333-98) The present invention relates to waveguide apparatus employed in propagation of electromagnetic wave energy at microwave frequencies and more particularly to an improved resonant window assembly for switching devices utilizing a gaseous discharge.

Prior art resonant window assemblies having an integral gaseous atmosphere contained within an encapsulated dielectric structure are described in U.S. Patent 2,791,720 issued May 7, 1957 to F. A. Lesch and 2,407,069 issued September 3, 1946 to M. D. Fiske. The advantage of such structures is that the gaseous atmosphere characteristics may be optimized for the arc loss and recovery time electrical parameters when the resonant window assembly is combined with waveguide structure to form a switching device. As a result of the encapsulated window structure improvement in recovery time has been achieved by the use of an electron capture gas such as water vapor in combination with conventional noble gaseous atmospheres. The addition of the water vapor, however, presents certain disadvantages in that oxidation of any exposed metallic surfaces is likely to occur and temperature extremes will result in drastic changes in the water vapor pressure thereby altering the gaseous atmosphere composition. Furthermore, water vapor will freeze at C. which prevents usage of such an additive for switching radar signals at lower operating temperatures.

An analysis of the gaseous discharge phenomena and in particular the deionization processes which determine the recovery time mechanism is necessary to appreciate the improvement disclosed in the present invention. In the conventional gaseous discharge switching of micro- Wave frequency energy, the intense ionization during the transmission cycle results in an electron density in the range of 101 to 1013 electrons/cc. dependent upon the incident power, gas composition, gas pressure and conguration of the gas container. The ionization is achieved by the increasing voltage gradient which results in rapid acceleration of electrons to provide elastic collisions and eventually inelastic collisions. Upon cessation of the transmitter energy, the recovery or deionization process must proceed by operation of three processes; namely attachment, recombination and diffusion.

In the switching tube art the attachment mechanism is the most prominently relied upon to control recovery time. In -this mechanism, the molecule attaches an electron to form a negative ion. Since water vapor has a high electron affinity it is universally employed as an additive. The fast deionization time achieved, however, has resulted in an increase in the arc loss or thermal energy dissipated within the gaseous discharge container. In many cases this thermal energy is sufliciently high to melt a dielectric material. It has been observed in the art, however, that with a pure dry noble gas such as argon, krypton or xenon the arc loss can be reduced by a factor of to l. The recovery time, however, would increase beyond the tolerable limits for radar usage since the deionization mechanism now relied upon is a simple diffuarent sion process. Inasmuch as the prior art container designs resulted in a relatively long diffusion path, the longer recovery times may be explained. It, therefore, is desirable to employ a pure noble gaseous atmosphere in a container configuration which provides a short diffusion path to result in a low arc loss and a short .recovery time in switching electromagnetic energy.

The present invention has for its primary object the provision of a resonant window assembly for gaseous discharge switching devices, said resonant Window assembly having improved electric characteristics of low arc loss and short recovery time.

A further object is the provision of an improved enclosure for a gaseous atmosphere for switching of high power electromagnetic energy.

A still further object is the provision of an improved enclosure for containing a gaseous discharge within a relatively small cross-sectional area approximating the mean free path of the gas molecule.

A feature of the present invention resides in the spacing within a gaseous atmosphere container being approximately equal to the mean free path of a selected pure noble gas contained therein. In accordance with the text Basic Data of Plasma Physics by Sanborn C. Brown, Technology Press and John Wiley and Sons, Inc., New York 1958 pages 1-18, 48-49 lthe diiusion length or gap dimension is defined by:

Eq. (1) A2=DT wherein D==dilusion coefficient, of gas, and r=recovery time;

hence, by transposing:

For parallel plate geometry the diifusion length is deiined by:

wherein L is distance of plate separation Now, by substituting Eq. 3 in Eq. 2 the following equa- In accordance with the present invention, therefore, as the distance L becomes smaller the recovery time will also become shorter. It is believed that with the short diffusion path the electrons traverse the gap rapidly and are neutralized by the adjacent plates.

The invention further permits the use of pure noble gases, heretofore not used in conventional gaseous discharge tube envelopes because of the long diffusion path which resulted in high recovery time values.

The calculation of the spacing of the plates to define the gas envelope has been experimentally determined to be approximately equal to the mean free path of a selected gas in accordance with the definition in the aforementioned text on pages 2 3:

with p being the actual pressure employed and T lis the electron temperature during ionization; and Pc is the average number of collisions that occur when an electron travels 1 cm. at a pressure lof 1 mm. at 0 C. In the practice of the invention, water vapor as an addltive gas in the known arjt` to facilitate the attachment mechanism maybe eliminated `since the'main feature ofY the present disclosure involves the diusion mechanism in a gas container having a critically short plate separation incorporated therein.

VOther objects, features Vand'advantages will be apparent after consideration of the following detailed specication and accompanying drawings, in which:

Fig. 1 is a side elevational view partially broken away;

Fig. 2 is a side elevational view of the inner box member; Y

Fig. 3 is a view'alongthe'line 3--3 in Pig. 2;'and

Fig. 4 is an exploded view of ,theillustrative embodiment showing the method of assembling the components.

Referring `to the drawings,fthe illustrative embodiment 1 comprises plural parallel metallic plate members 2 and 3 spaced apart a distance approximating the mean free path 'of a gas Ymolecule to' delinea very narrow gap 4. Each'parallel plate member is provided with a centrally located resonant aperture covered by a dielectric window member 5 and 6. To support the parallel plate members in the desired position and control the critical spacing therebetweeneach'plate member isjoined to a short length of rectangular waveguide. Hence, plate member 2 will be hermetically'sealed to waveguide 7, while plate member 3 is joined at one endto waveguide 8 of'larger overall dimensions. `Hence waveguide 7 will be positioned to snugly engage the inner walls of waveguide 8 in a telescoping arrangement. A mounting ange 9 secured as shown provides means for connecting the window assembly to adjacent waveguide structure when the assembly is utilized to form any lof-the switching devices. Waveguide 7 is provided with wall structure resembling a notch along a narrow side to thereby dene with waveguide 8, a passageway 11 for exhausting and introduction of the gas atmosphere. Y

In the method of assembling the resonant windo-w assembly .the outer surface 12 .of window 5 should desirably be coextensive with the surface of plate member. It may be necessary, therefore, to grind the dielectric material to achieve the desired ilatness. Similarly the excess dielectric material exposed on the inner surface of window 6 must be removed to achieve a flat surface as at 13. With appropriate jigging and measurement fixtures waveguide 7 is inserted within'waveguide 8 until gap 4 is defined. The distance d between the parallel Vplate members will approximate the mean free path which by definition is the average distance an electron travels before a collison with a gas atom. Experimental values have been determined from .005 to .05 inch for vario-us noble gases and pressures. For an illustrative example a filling of argon gas at a pressure of 7 millimeters of mercury and a gap spacing of .012 inch resulted in a recovery time of approximately microseconds. The arc loss characteristic was below the sensitivity of conventional measuring techniques.

After insertion-of waveguide 7 within waveguide 8 and adjustment of the gap spacing the outer ends of the waveguides may be joined together by fusing of the 'metaL as at'14 by the process referred to as Heli-weld to form a Vacuum tight seal. Such a sealing process assures a considerably improved hermetic seal over prior art soldering techniques. Afeature of the present window assembly resides in the fact that the intense arc utilized in the welding process is removed from the dielectric window area thereby averting any melting or cracking.

The assembly is evacuated and filled with a gaseous atmosphere by means of tubulation 15 supported by a triangular metallic member 16. Tubulation 15 communicates with passageway 11 to provide access to the gap between the parallel plate members. Other structure for exhausting and filling will be readily apparent and may be positioned so as to extend within the open area 17 defined by waveguide 7. It ywill also `-bepossible to provide a gas reservoir within the area 17 by means of a suitable dielectric member ina manner well-known in the commonly found in the` art. YThe window assembly is simply secured to the necessary waveguide structure by soldering or other mechanical means. The window as- Y sembly may Valso be easily. replaced whenever necessary without the costly replacement 'of the entire switching tube. Among the various devices madepossible by this arrangement are: transmit-receive; anti-transmit-receive; pre-transmit-receive and attenuator tubes.

Y The illustrative embodiment has beendeseribed with reference to Va rectangularjstructure, however, circular waveguide congurations 'may be adapted similarly.

What is claimed-is: o Y

1.A resonant window vassembly for electromagnetic wave energy gaseous discharge'switching'devices comprising plural spaced parallel plate members having resonant apertures enclosed by material previous to electromagnetic energy, waveguide means secured to yeach of said plate members with one waveguide being inserted within another to thereby retain thespacing between said plates, said waveguide means being hermetically sealed together to define a vacuum tight enclosure, a filling of a pure noble gas within said enclosure at reduced pressures, the spacing between said platemembers lbeing'critically short to equal the mean free path of a gas molecule of the selected gas.

2. A'resonant window'assembly for electromagnetic wave-energy gaseous discharge switching devices comprising plural narrowly spaced parallel plate members defining therebetween a -gap of critically short dimensions. equal to the rnean free path of Ia gas molecule of a gas, each of said plate members Vhaving a centrally located resonant aperture with a :dielectric window positioned therein, ltheY spacing between said plate members being retained by means V.of mounting said plate members at the ends of hollow waveguide sections with one of said sections positioned within Vthe other Vin -telescoping arrangement, the opposite-ends -of `'said waveguide sections being hermetically sealed together to y'form a vacuum tight enclosure, and a filling of a Vpure noble gas under :re-- duced pressure contained within said enclosure.

3. A resonant window assembly .for mounting electromagnetic waveguide structure comprising plural narrowly spaced parallel plate members defining therebetween a Vgap -cf critically short dimensions, each of said platemernbers having a centrally located rectangular laperture with a dielectric window positioned therein, -a section of hollow rectangular waveguide secured to each plate member with one of said sections being positioned within the otherin telescoping arrangement, the outer endsl of said waveguide sections being Vhermetically sealed together to forma vacuum tight enclosure, a gas selected from the'pure noble gases contained within said enclosure under reduced pressure, the dimensions of said gap being equal to .the mean free Vpath Yof a gas molecule of the selected gas. v

References Cited in the le of this patent UNrTED STAT-Es PATENTS 2,407,069 VFiske Sept. :3, .1946 2,745,014 Norton May 8 1956 2,791,720 Y f VLesch f.... f May 7, 1957 2,832,053 Y Dicke Apr. `22,V 1958 `2,866,130 Ghenw...Y Dec. 23, Y.195.8 2,903,623 Walker Sept. 8, v1.959 

